With liquid crystal displays (LCD) having rapidly spread in products such as TVs, cell phones, etc., as a result of an increase in screen size and higher functions, a higher performance display is demanded. In addition, simplification of the fabrication process by incorporating a large-scale integration (LSI) driver circuit, etc., around an LCD into the LCD has been demanded.
To meet these demands, the key to improving the LCD is the performance of thin-film transistors (TFT). In addition, thin-film transistors (TFT) are expected to serve as a technique for realizing system-on-panel or the like. The TFT has a function, etc., as a switch for charging various pixels of the LCD in response to a driver circuit, and at present, in many cases, the TFT is formed of an amorphous film such as amorphous silicon set on a transparent substrate such as a glass substrate.
However, the TFT formed of an amorphous film is low in electron mobility and hardly meets the demand for higher performance, so that various attempts have been made to try to improve the amorphous film.
Therefore, for a channel layer of the TFT, a silicon film with high quality has been demanded.
One of the more typical trial improvements is substitution of the amorphous silicon with polysilicon. Due to polycrystallization of silicon, the electron mobility is increased from 0.1 to 0.2 cm2/Vs to 10 to 500 cm2/Vs. In this case, the larger the crystal grains, the fewer the barriers of the crystal grain boundary are in the path of electrons, so that it is desirable to acquire crystal grains as large as possible. The electron mobility of polysilicon with a large grain size (several micrometers) is equivalent to that of single crystal (500 to 700 cm2/Vs).
As a typical method of crystallizing amorphous silicon, there is a method using annealing according to a solid-phase crystallization method. This is a method in which random silicon bonds are once cut and silicon atoms are rearranged. In this method, generally, amorphous silicon must be heated to about 600 degrees C., and this deteriorates the material, and it becomes difficult to use an inexpensive glass substrate.
Therefore, a method of crystallizing amorphous silicon at a low temperature (not more than 550 degrees C.) has been demanded, and this has been variously researched. For using a glass substrate, crystallization at a low temperature is desirable.
In various studies, it is thought that the use of metal, etc., as a core of crystallization shows promise in promoting crystallization at a low temperature. Particularly, nickel silicide (NiSi2) has a lattice constant close to that of silicon, and has less distortion in use, so that it is considered as most promising.
As a study on the use of nickel, a metal imprint method is conventionally known. In the metal imprint method, a tip-array coated with a nickel thin film is pressure-bonded to an amorphous silicon film and silicon is crystallized by means of solid-phase growth by using a minute amount of metal mark at a position where the tip came into contact with as a crystal core, and the amount of nickel to be transferred changes depending on the method of applying a force, so that the amount cannot be strictly controlled.
As another technique using nickel, a method in which a film of nickel or the like is overlaid on an amorphous silicon film by means of sputtering or electron beam deposition and a method in which nickel or the like is introduced by means of electroless plating, selective chemical deposition, or ion implantation have been proposed (for example, refer to Patent document 1). However, none of these supply only the necessary amount of nickel by properly controlling the additional amount of nickel, but remove an excess supply later.
[Patent document 1] Japanese Unexamined Patent Publication No. H11-87242